1. Field of the Invention
The present invention is related to an actuator for driving valves such as butterfly valves and boll valves, which convert a reciprocating motion of pistons into a rotating motion, which then transmits the motion to the valve driving shaft. In particular, the invention is related to an actuator for driving valves that includes two of the aforesaid pistons and drives each piston in the direction of a shaft center by pressing with fluid pressure supplied into cylinder rooms provided at both sides of the pistons.
2. Description of the Related Art
As shown in FIG. 1, in a conventional rotary actuator R0 for butterfly valve driving, a four-port switching valve 104 intervenes between a pressure source 101 such as pumps, a tank 102 and a rotary actuator 103. In this condition, the working fluid, such as pressure oil or pressure air that is discharged from a pressure source 101, goes into a left side cylinder room 106 of a left side cylinder 109 via a flow control valve 105, and a portion of the working fluid goes into a left side cylinder room 108 of a right side cylinder 110 via a pipe 107, and thereby pressing pistons 111 and 112 intruded into the cylinders 109 and 110 in the direction of an arrow X. This sliding movement of the pistons 111 and 112 releases the fluid discharged from cylinder rooms 113 and 114 into the tank 102 from the four-port switching valve 104 via a flow control valve 115.
In order to move the piston in the reverse direction, the four-port switching valve 104 needs to be switched in the direction of arrow Y. According to this switching of the fluid path, the pistons 111 and 112 and a piston rod 116 that connects the both pistons, perform reciprocating motion as a unit.
In the meantime, a trunnion pin 118, which is implanted in a trunnion 117 fittingly fixed to the piston rod 116, is inserted into a yoke groove 122 of a yoke 121 fixed on a valve driving shaft 120 that is rotatably placed on a cylinder body 119, and when the pistons 111 and 112 perform the reciprocating motion, thereby sliding on the yoke groove 122 as pushing its side walls. Accordingly, the yoke 121 swings, and the butterfly valve connected to the valve driving shaft 120 rotates.
The foregoing mechanism that converts the reciprocating motion into the rotating motion by means of yokes is called “scotch yoke mechanism”, and the yoke 121 equipped with the trunnion pin 118 and the yoke groove 122 is an example of the converters.
The above-described rotary actuator R0 for valve driving is disclosed in Japanese Utility Models Publications No. S63-32961 and H06-73405.
The above-mentioned conventional rotary actuator R0, which uses two pistons for rotating one valve driving shaft 120, has a benefit of obtaining larger driving forth compared with the conventional invention that uses only one piston disclosed in Japanese Patent Publication No. 2000-104706.
However, for example, the conventional actuator for valve driving disclosed in Japanese Utility Model Publication No. S63-32961 distributes the pressure fluid from one cylinder room to the other cylinder room by using such as external piping 107, thereby causing not only the enlargement of the actuator itself but also the increased material cost occurred from the additional piping.
Also, in the case of Japanese Utility Model Publication No. H06-73405, although the actuator is not enlarged since the piping of compressed air is formed inside of the rod corresponding to the above-mentioned piston rod 116, it does not still form a preferable structure since the hollow piston rod that contains the piping inside degrades the strength that the piston rod requires.
Naturally, although the above problem will be solved if the size of the piston rod is enlarged, it causes another problems of reducing the pressure receiving area of the piston, or enlarging the actuator in an attempt to secure sufficient pressure receiving area.
Therefore, the purpose of the present invention is to provide an actuator for valve driving, which comprises a piping structure between cylinders without enlarging its size, without increasing the cost, and without reducing the pressure receiving area of the pistons.